Heat the oil in a pan and brown the onion and celery.
Add beetroot and cabbage, then stock, vinegar, salt, pepper and bay leaf.
Bring to the boil, cover, then leave to simmer for an hour.
Allow to cool a little, remove bay leaf, then blend mixture in a food processor.
Add water, edjust seasoning and reheat.
Serve in bowls and add a dollop of yoghurt.